Low resistance tunneling magnetoresistive sensor with natural oxidized double MgO barrier

ABSTRACT

A high performance TMR sensor is fabricated by incorporating a tunnel barrier having a Mg/MgO/Mg configuration. The 4 to 14 Angstroms thick lower Mg layer and 2 to 8 Angstroms thick upper Mg layer are deposited by a DC sputtering method while the MgO layer is formed by a NOX process involving oxygen pressure from 0.1 mTorr to 1 Torr for 15 to 300 seconds. NOX time and pressure may be varied to achieve a MR ratio of at least 34% and a RA value of 2.1 ohm-um 2 . The NOX process provides a more uniform MgO layer than sputtering methods. The second Mg layer is employed to prevent oxidation of an adjacent ferromagnetic layer. In a bottom spin valve configuration, a Ta/Ru seed layer, IrMn AFM layer, CoFe/Ru/CoFeB pinned layer, Mg/MgO/Mg barrier, CoFe/NiFe free layer, and a cap layer are sequentially formed on a bottom shield in a read head.

RELATED PATENT APPLICATION

This application is related to the following: Docket # HMG05-043, Ser.No. ______, filing date ______; assigned to a common assignee.

FIELD OF THE INVENTION

The invention relates to a high performance tunneling magnetoresistive(TMR) sensor in a read head and a method for making the same, and inparticular, to a method of forming a MgO barrier between a pinned layerand free layer that improves the MR ratio and tunnel barrier uniformitywhile achieving a low resistance×area (RA) value.

BACKGROUND OF THE INVENTION

A TMR sensor serves as a memory element in magnetic devices such asMagnetic Random Access Memory (MRAM) and a magnetic read head. In FIG.1, a TMR sensor 6 is shown as a stack of layers that has a configurationin which two ferromagnetic layers are separated by a thin non-magneticdielectric layer. In a magnetic read head, the TMR sensor 6 is formedbetween a bottom shield 5 and a top shield 14. The bottom (seed) layer 7in the TMR sensor 6 is generally comprised of one or more seed layersthat promote a smooth and dense crystal growth in overlying layers.Above the seed layer 7 is an anti-ferromagnetic (AFM) pinning layer 8and a first ferromagnetic layer that is a “pinned” layer 9 on the AFMlayer. The thin tunnel barrier layer 10 above the pinned layer 9 isgenerally comprised of a dielectric material such as AlOx that may beformed by first depositing an Al layer and then performing an in-situoxidation. The tunnel barrier layer 10 must be extremely uniform inthickness and oxidation state since small AlOx thickness variations orslight oxidation differences result in large variations in resistancethat degrade device performance. A ferromagnetic “free” layer 11 isformed on the tunnel barrier layer 10 and is typically less than 50Angstroms thick. At the top of the TMR element is a cap layer 12. In aMRAM, the TMR sensor is formed between a bottom conductor and a topconductor

The MTJ stack in FIG. 1 has a so-called bottom spin valve configuration.Alternatively, an MTJ stack may have a top spin valve configuration inwhich a free layer is formed on a seed layer followed by sequentiallyforming a tunnel barrier layer, a pinned layer, AFM layer, and a caplayer.

The pinned layer 9 has a magnetic moment that is fixed in the ydirection by exchange coupling with the adjacent AFM layer 8 that isalso magnetized in the y direction. The free layer 11 has a magneticmoment that is either parallel or anti-parallel to the magnetic momentin the pinned layer. The tunnel barrier layer 10 is so thin that acurrent through it can be established by quantum mechanical tunneling ofconduction electrons. The magnetic moment of the free layer may switchin response to external magnetic fields generated by passing a currentthrough the bottom shield 5 and top shield 14. It is the relativeorientation of the magnetic moments between the free and pinned layersthat determines the tunneling current and therefore the resistance ofthe tunneling junction. When a sense current 15 is passed from the topshield 14 to the bottom shield 5 (or top conductor to bottom conductorin a MRAM device) in a direction perpendicular to the planes of the TMRlayers (CPP designation), a lower resistance is detected when themagnetization directions of the free and pinned layers are in a parallelstate (“1” memory state) and a higher resistance is noted when they arein an anti-parallel state or “0” memory state. Alternatively, a TMRsensor may be configured as a current in plane (CIP) structure whichindicates the direction of the sense current.

A TMR sensor is currently the most promising candidate for replacing agiant magnetoresistive (GMR) sensor in upcoming generations of magneticrecording heads. An advanced TMR sensor may have a cross-sectional areaof about 0.1 microns×0.1 microns at the air bearing surface (ABS) planeof the read head. The advantages of a TMR sensor are a higher MR ratioand the preference for CPP geometry for high recording density. A highperformance TMR sensor requires a low RA (area×resistance) value, highMR ratio, a soft free layer with low magnetostriction (λ), a strongpinned layer, and low interlayer coupling through the barrier layer. TheMR ratio is dR/R where R is the minimum resistance of the TMR sensor anddR is the change in resistance observed by changing the magnetic stateof the free layer. A higher dR/R improves the readout speed. For highrecording density or high frequency applications, RA must be reduced toabout 1 to 3 ohm-um². As a consequence, MR ratio drops significantly. Tomaintain a reasonable signal-to-noise (SNR) ratio, a novel magnetictunneling junction (MTJ) with a MR ratio higher than that provided by aconventional AlOx barrier layer is desirable.

A very high MR ratio has been reported by Yuasa et. al in “Giantroom-temperature magnetoresistance in single crystal Fe/MgO/Fe magnetictunnel junctions”, Nature Materials 3, 868-871 (2004) and is attributedto coherent tunneling. Parkin et al in “Giant tunnelingmagnetoresistance at room temperature with MgO (100) tunnel barriers”,Nature Materials 3, 862-867 (2004) demonstrated that an MR ratio ofabout 200% can be achieved with epitaxial Fe(001)/MgO(001)/Fe(001) andpolycrystalline FeCo(001)/MgO(001)/(Fe₇₀Co₃₀)₈₀B₂₀ MTJs at roomtemperature. In addition, Djayaprawira et. al described a high MR ratioof 230% with advantages of better flexibility and uniformity in “230%room temperature magnetoresistance in “CoFeB/MgO/CoFeB magnetic tunneljunctions”, Physics Letters 86, 092502 (2005). However, RA values in theMTJs mentioned above are in the range of 240 to 10000 ohm-um² which istoo high for read head applications. Tsunekawa et. al in “Gianttunneling magnetoresistance effect in low resistanceCoFeB/MgO(001)/CoFeB magnetic tunnel junctions for read headapplications”, Applied Physics Letters 87, 072503 (2005) found areduction in RA by inserting a DC-sputtered metallic Mg layer between abottom CoFeB layer and rf-sputtered MgO. The Mg layer improves thecrystal orientation of the MgO(001) layer when the MgO(001) layer isthin. The MR ratio of CoFeB/Mg/MgO/CoFeB MTJs can reach 138% at RA=2.4ohm-um². The idea of metallic Mg insertion was initially disclosed byLinn in U.S. Pat. No. 6,841,395 to prevent oxidation of the bottomelectrode (CoFe) in a CoFe/MgO(reactive sputtering)/NiFe structure.

Although a high MR ratio and low RA have been demonstrated in MTJshaving a MgO barrier layer, there are still many issues to be resolvedbefore such configurations can be implemented in a TMR sensor of a readhead. For example, the annealing temperature needs to be lower than 300°C. for read head processing, and rf-sputtered MgO barriers make controlof RA mean and uniformity more difficult than with conventionalDC-sputtered and subsequently naturally oxidized AlOx barriers.Moreover, a CoFe/NiFe free layer is preferred over CoFeB for low λ andsoft magnetic properties but when using a CoFe/NiFe free layer incombination with a MgO barrier, the MR ratio will degrade to very nearthat of a conventional AlOx MTJ. Thus, a TMR sensor is needed thatincorporates a MgO barrier without compromising any desirable propertiessuch as high MR ratio, a low RA value, and low magnetostriction.

A three step barrier layer formation process for a TMR sensor isdescribed in U.S. Pat. No. 6,841,395 and involves sequentiallydepositing a Mg layer and an oxygen doped Mg film on a ferromagneticlayer and then performing an oxygen treatment. In U.S. Pat. No.6,828,260, a UV light is used to irradiate a MgO tunnel barrier layerthrough a transparent overlayer and thereby activate unreacted oxygen inthe barrier layer to react with Mg and form a uniformly oxygenatedtunnel barrier layer.

A barrier layer comprised of TiO_(X)N_(Y) and MgO is disclosed in U.S.Pat. No. 6,756,128. In U.S. Pat. No. 6,737,691, a tunneling barrierlayer such as MgO is described with a thickness of <10 Angstroms. Nocomposite barrier layer is disclosed. In U.S. Pat. No. 6,347,049,MgO/Al₂O₃/MgO and Al₂O₃/MgO/Al₂O₃ are disclosed as tunnel barrier layershaving low RA values.

A natural oxidation process is used to form a tunnel barrier layercomprised of a single oxide layer in U.S. Pat. No. 6,887,717, U.S. Pat.No. 6,826,022, and in U.S. Pat. No. 6,819,532.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide a TMR sensor with anatural oxidized MgO barrier layer to improve the uniformity of thebarrier thickness and oxidation compared with rf-sputtered or reactivesputtered MgO barriers.

A second objective of the present invention is to provide a TMR sensorhaving a substantially higher MR ratio than realized with sensorscomprised of an AlOx barrier layer while achieving a low RA value andlow magnetostriction required for advanced read head applications.

A further objective of the present invention is to provide a method offorming a TMR sensor that satisfies the first and second objectives andis cost effective.

According to a first embodiment, these objectives are achieved byforming a TMR sensor on a suitable substrate such as a bottom shield ina read head. The TMR sensor may have a bottom spin valve configurationcomprised of a seed layer, AFM layer, and pinned layer which are formedsequentially on the bottom shield. The pinned layer preferably has anAP2/coupling/AP1 configuration in which the coupling layer is Ru and theAP1 layer is made of CoFeB or a CoFeB/CoFe composite. A key feature isthat the barrier layer formed on the AP1 layer has a composite structurecomprised of a Mg/MgO/Mg stack. The Mg layers are formed by a DCmagnetron sputtering method and the MgO layer is made by a naturaloxidation process of the lower Mg layer. Above the tunnel barrier layeris formed a free layer and then a cap layer as the uppermost layer inthe TMR sensor. A CoFe/NiFe free layer is preferred for lowmagnetostriction.

Typically, a TMR stack of layers is laid down in a sputtering system.All of the layers may be deposited in the same sputter chamber. However,the natural oxidation process on the lower Mg layer to form a MgO layerthereon is preferably done in an oxidation chamber within the sputteringsystem. The TMR stack is patterned by a conventional method prior toforming a top shield on the cap layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a conventional TMR sensor thatis formed between a bottom shield and a top shield in a magnetic readhead.

FIG. 2 is a cross-sectional view depicting a TMR stack having a barrierlayer comprised of a middle MgO layer and upper and lower Mg layersaccording to one embodiment of the present invention.

FIG. 3 is a process flow that depicts the steps involved in forming aMg/MgO/Mg barrier layer according to one embodiment of the presentinvention.

FIG. 4 is a cross-sectional view of a TMR read head having a TMR sensorinterposed between a top shield and bottom shield and formed accordingto the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a high performance TMR sensor having a barrierlayer comprised of a Mg/MgO/Mg stack and a method for making the samewherein the MgO layer may be formed by a natural oxidation process ofthe lower Mg layer. While the exemplary embodiment depicts a TMR sensorin a read head, the present invention may be employed in other devicesbased on a tunneling magnetoresistive element such as MRAM structures.The TMR sensor may have a bottom spin valve, top spin valve, ormultilayer spin value configuration as appreciated by those skilled inthe art. The drawings are provided by way of example and are notintended to limit the scope of the invention. For example, the drawingsare not necessarily drawn to scale and the relative sizes of variouselements may differ compared with those in an actual device.

Referring to FIG. 2, a portion of a partially formed TMR read head 40 ofthe present invention is shown from the plane of an air bearing surface(ABS). There is a substrate 21 that in one embodiment is a bottom leadotherwise known as a bottom shield (S1) which may be a NiFe layer about2 microns thick that is formed by a conventional method on asubstructure (not shown). It should be understood that the substructureis typically comprised of a first gap layer disposed on a wafer made ofAlTiC, for example.

A TMR stack 31 is formed on the substrate 21 and in the exemplaryembodiment has a bottom spin valve configuration wherein a seed layer22, AFM layer 23, and a pinned layer 24 are sequentially formed on thesubstrate as the bottom portion of the TMR stack. The seed layer 22which has a thickness of about 10 to 100 Angstroms is preferably a Ta/Rucomposite but Ta, Ta/NiCr, Ta/Cu, or Ta/Cr may be used, instead. Theseed layer 22 serves to promote a smooth and uniform grain structure inoverlying layers. Above the seed layer 22 is an AFM layer 23 used to pinthe magnetization direction of the overlying pinned layer 24. The AFMlayer 23 has a thickness from 40 to 300 Angstroms and is preferablycomprised of IrMn. Optionally, one of MnPt, NiMn, OsMn, RuMn, RhMn,PdMn, RuRhMn, or MnPtPd may be employed as the AFM layer.

The pinned layer 24 may have a synthetic anti-parallel (SyAP)configuration (not shown) represented by AP2/Ru/AP1. The AP2 layer(outer pinned layer) is formed on the AFM layer 23 and may be made ofCoFe with a composition of about 10 atomic % Fe and with a thickness ofabout 10 to 50 Angstroms. The magnetic moment of the AP2 layer is pinnedin a direction anti-parallel to the magnetic moment of the AP1 layer. Aslight difference in thickness between the AP2 and AP1 layers produces asmall net magnetic moment for the SyAP pinned layer 24 along the easyaxis direction of the TMR sensor to be patterned in a later step.Exchange coupling between the AP2 layer and the AP1 layer is facilitatedby a coupling layer that is preferably comprised of Ru with a thicknessof from 3 to 9 Angstroms although Rh or Ir may be used instead of Ru. Ina preferred embodiment, the AP1 layer (inner pinned layer) is aCo_(1-X—Y)Fe_(X)B_(Y) layer wherein x is from about 5 to 95 atomic % andy is from about 5 to 30 atomic % and is formed on a Ru coupling layerwhich is about 7.5 Angstroms thick. The Co_(1-X—Y)Fe_(X)B_(Y) layer hasa thickness from about 10 to 80 Angstroms. The Co_(1-X—Y)Fe_(X)B_(Y)layer is amorphous and provides a more uniform pinned layer 24 thanwould result if a CoFe layer, for example, were employed as the AP1layer. Optionally, the AP1 layer may be a multilayer structure such asCoFeB/Co_(1-V)Fe_(V) wherein v is from 10 to 90 atomic % and theCo_(1-V)Fe_(V) thickness is from about 5 to 20 Angstroms with the totalAP1 layer thickness from 10 to 80 Angstroms. In this optionalconfiguration, the uniformity of the pinned layer may not be as high asin the preferred embodiment, but the TMR properties such as MR ratio andRA value are similar to the preferred embodiment.

Note that “inner pinned layer” is meant to indicate the portion of thepinned layer that is closest to the barrier layer and “outer pinnedlayer” is meant to signify the portion of the pinned layer farthest fromthe barrier layer. In the exemplary embodiment that features a bottomspin valve configuration, the barrier layer 28 having a Mg/MgO/Mg stackof layers 25-27 is formed on the AP1 portion of the pinned layer 24. Ina top spin valve configuration, the TMR stack according to the presentinvention would involve the sequential deposition of a seed layer, freelayer, Mg/MgO/Mg barrier layer, pinned layer with an AP1/Ru/AP2configuration, an AFM layer, and a cap layer on the substrate. In a topspin valve, the AP1 layer (inner pinned layer) is disposed on the upperMg layer in the Mg/MgO/Mg stack.

A key feature of the present invention according to the preferredembodiment is the formation of a barrier layer 28 having a Mg/MgO/Mgconfiguration on the inner (AP1) portion of the pinned layer 24. Aprocess flow diagram is provided in FIG. 3. The first or lower Mg layer25 is preferably between 4 and 14 Angstroms thick and is deposited (step50) on a ferromagnetic layer in a DC sputtering chamber of a sputteringsystem such as an Anelva C-7100 sputter deposition system which includesultra high vacuum DC magnetron sputter chambers and at least oneoxidation chamber. In one aspect relating to a bottom spin valveconfiguration, the ferromagnetic layer in step 50 is the pinned layer24. Alternatively, in a top spin valve configuration (not shown), theferromagnetic layer is a free layer. Typically, the sputter depositionprocess involves an argon sputter gas and a base pressure between 5×10⁻⁸and 5×10⁻⁹ torr. Each sputter chamber has multiple targets which are lowpressure discharge cathodes. A lower pressure enables more uniform filmsto be deposited.

The second step in the barrier layer formation sequence is a naturaloxidation (NOX) process (step 51) of the first Mg layer 25 to form a MgOlayer 26 thereon. The NOX process is performed in an oxidation chamberof the sputter deposition system using an oxygen pressure of 0.1 mTorrto 1 Torr for about 15 to 300 seconds. In the exemplary embodiment, noheating or cooling is applied to the oxidation chamber during the NOXprocess. The resulting MgO layer 26 has a thickness of about 5 to 12Angstroms. It should be noted that oxygen pressures below 0.1 mTorr arenot recommended because of tool limitations. For an oxygen pressure >1Torr, a low RA value (RA <5 ohm-uM²) cannot be obtained with areasonable NOX time of greater than about 10 seconds. In a relatedpatent application HMG05-043 which is herein incorporated by referencein its entirety, a tunneling barrier layer comprised of an upper MgOlayer made by an NOX process and a lower MgO layer formed by a ROXprocess is described.

The present invention anticipates that a Mg/MgO/Mg barrier layer 28according to a second embodiment of the present invention could beformed by depositing the MgO layer 26 on the first Mg layer 25 with arf-sputtering or reactive sputtering method. It should be understoodthat the performance of a TMR sensor fabricated with a barrier layercomprised of sputtered MgO will not be as desirable as one madeaccording to the preferred embodiment of this invention. For example,the inventors have observed that the final RA uniformity (1σ) of 0.6 umcircular devices is more than 10% when the MgO tunnel barrier layer isrf-sputtered and less than 3% when the MgO tunnel barrier is formed byDC sputtering a Mg layer followed by a NOX process.

Referring again to FIG. 3, the partially formed TMR stack 31 is returnedto a sputter deposition chamber in the sputter deposition system (step52) and a second Mg layer 27 is deposited on the MgO layer 26 by a DCsputtering process. The second Mg layer 27 has a thickness between 2 and8 Angstroms and serves to protect the subsequently deposited free layerfrom oxidation. It is believed that excessive oxygen accumulates at thetop surface of the MgO layer 26 as a result of the NOX process and thisoxygen can oxidize a free layer that is formed directly on the MgOportion of the barrier layer. Likewise, a pinned layer formed directlyon the MgO portion of a barrier layer in a top spin valve configurationwould be subject to oxidation and a Mg/MgO/Mg barrier stack of thepresent invention would protect the pinned layer. Note that the RA andMR ratio for the TMR sensor 31 may be adjusted by varying the thicknessof the two Mg layers 25, 27 and by varying the natural oxidation timeand pressure. For example, a thicker MgO layer resulting from longeroxidation time and/or higher pressure would increase the RA value.

Returning to FIG. 2, a free layer 29 is formed on the barrier layer 28and is preferably comprised of Co_(1-W)Fe_(W)/Ni_(1-Z)Fe_(Z) wherein wis from about 10 to 90 atomic % and z is from about 5 to 70 atomic %.Optionally, the free layer 29 may be a multilayered structure of alloysincluding at least two of the elements Co, Fe, Ni, and B. The free layer29 has a thickness in the range of 10 to 90 Angstroms. Above the freelayer is a cap layer 30 that may have a Ru/Ta/Ru or Ru/Zr/Ruconfiguration, for example. In one embodiment, the free layer 29 and caplayer 30 are deposited in the same sputtering chamber as the layers22-25 and second Mg layer 27. Optionally, all of the sputter depositedlayers in the TMR stack 31 may be deposited in more than one sputterchamber in the same sputter deposition system or mainframe.

Once the TMR stack 31 is complete, the partially formed read head 40 maybe annealed in a vacuum oven within the range of 250° C. to 300° C. witha 8000 Oe applied magnetic field for about 2 to 10 hours to set thepinned layer and free layer magnetization directions. It should beunderstood that in some cases, depending upon the time and temperatureinvolved in the anneal process, the Mg/MgO/Mg tunnel barrier may becomea uniform MgO tunnel barrier layer as unreacted oxygen diffuses into theadjacent Mg layers.

Next, the TMR stack 31 is patterned by following a conventional processsequence. For example, a photoresist layer (not shown) may be applied onthe cap layer 30. After the photoresist layer is patterned, an ion beametch (IBE) or the like is used to remove underlying layers in the TMRstack that are exposed by openings in the photoresist layer. The etchprocess stops on the bottom shield 21 to give a TMR sensor 31 a with atop surface 31 b and sidewalls 31 c as shown in FIG. 4. Thereafter, aninsulating layer 32 may be deposited along the sidewalls of the TMRsensor 31 a. The photoresist layer is subsequently removed by a lift offprocess.

A top lead otherwise known as a top shield 33 is then deposited on theinsulating layer 32 and TMR element 31 a. Similar to the bottom shield21, the top shield 33 may also be a NiFe layer about 2 microns thick.The TMR read head 40 may be further comprised of a second gap layer (notshown) disposed on the top shield 33.

An experiment was conducted to demonstrate the improved performanceachieved by implementing a Mg/MgO/Mg barrier layer in a TMR sensor. ATMR stack of layers was fabricated according to the exemplary embodimentdescribed previously and has aTa/Ru/IrMn/CoFe/Ru/FeCoB/Mg/MgO/Mg/CoFe/NiFe/cap configuration. TheMg/MgO/Mg barrier is comprised of a 7 Angstrom thick lower Mg layer thatwas subjected to a NOX process and a 3 Angstrom thick upper Mg layer.The thicknesses in Angstroms of the other layers are given inparentheses: Ta(20)/Ru(20) seed layer; IrMn (70) AFM layer; CoFe (19)outer pinned layer; Ru (7.5) coupling layer; Fe₅₆Co₂₄B₂₀ (20) innerpinned layer; CoFe(10)/NiFe(40) free layer; and Ru(10)/Ta(60) cap layer.The TMR stack was formed on a NiFe shield and was annealed under vacuumat 280° C. for 5 hours with an applied field of 8000 Oe. The advantagesof the present invention are that a high MR ratio of greater than 30%can be achieved simultaneously with a low RA value (<5 ohm-um²) and lowmagnetostriction which is a significant improvement over conventionalTMR sensors (MTJs) based on AlOx barrier layers. In this example, a MRratio of 34% was achieved with a magnetostriction of 2×10⁻⁶ and a RAvalue of 2.1 ohm-um² on a 0.6 um circular device. It should beunderstood that the magnetostriction value may be adjusted by varyingthe thickness or composition of the CoFe and NiFe layers in the freelayer.

The inventors have previously fabricated a similar TMR sensor with thesame configuration as in the aforementioned experiment except the upperMg layer in the barrier layer was omitted. In other words, a single MgObarrier layer was fabricated rather than a double MgO barrier layer asin the present invention. As a result, the MR ratio decreased to about10% at an RA value of 2 ohm-uMm². Thus, the combination of high MR ratioand low RA is realized only by implementing the additional Mg layer inthe Mg/MgO/Mg barrier as disclosed herein. Furthermore, the Mg/MgO/Mgbarrier layer provides a higher MR ratio at low RA values than realizedwith conventional AlOx barrier layers. As mentioned earlier, the secondMg layer is advantageously used to prevent oxidation of the adjacentfree layer which in the exemplary embodiment is a CoFe layer. Thoseskilled in the art will appreciate that other ferromagnetic materialsthat serve as a free layer (or pinned layer) will likewise be protectedfrom oxidation by the second Mg layer in the tunnel barrier.

Another advantage of the present invention is that the Mg/MgO/Mg tunnelbarrier is cost effective. Compared with a process previously practicedby the inventors, the TMR sensor disclosed herein involves only anadditional Mg layer which does not require any new sputtering targets orsputter chambers. Moreover, the low temperature anneal enabled by theCoFe/NiFe free layer means that the annealing process can be kept thesame as for GMR sensors that are currently produced. Therefore, there isno change in process flow and related processes compared with currentmanufacturing schemes.

Yet another advantage is that the MgO layer in the present invention hasa more uniform thickness and oxidation state because it is formed by anatural oxidation method rather than by a sputtering process. As aresult, the tunneling resistance of the barrier layer is controlledwithin tighter limits and better control leads to higher performance.

While this invention has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this invention.

1. A method of forming a tunnel barrier layer in a tunnelingmagnetoresistive (TMR) sensor, comprising: (a) depositing a first Mglayer on a ferromagnetic layer; (b) performing a natural oxidation (NOX)process on said first Mg layer to form a MgO layer thereon; and (c)depositing a second Mg layer on said MgO layer.
 2. The method of claim 1wherein said ferromagnetic layer is a first ferromagnetic layer and themethod is further comprised of depositing a second ferromagnetic layeron said second Mg layer.
 3. The method of claim 2 wherein the first andsecond Mg layers and the first and second ferromagnetic layers aredeposited by a DC sputtering process in one or more chambers of asputtering system.
 4. The method of claim 3 wherein the NOX process isperformed in an oxidation chamber of said sputtering system, saidoxidation chamber is different than said one or more chambers of thesputtering system.
 5. The method of claim 1 wherein the NOX process isperformed in an oxidation chamber with an oxygen pressure between about0.1 mTorr and 1 Torr for a period of about 15 to 300 seconds.
 6. Themethod of claim 1 wherein the first Mg barrier has a thickness of about4 to 14 Angstroms and the second Mg layer has a thickness from about 2to 8 Angstroms.
 7. The method of claim 1 wherein the TMR sensor isformed on a bottom shield in a magnetic read head and is comprised of astack of layers in which a seed layer, an anti-ferromagnetic (AFM)layer, a pinned layer, said barrier layer, a free layer, and a cap layerare sequentially formed on the bottom shield.
 8. The method of claim 1wherein the first Mg layer is deposited on a ferromagnetic layer that isa synthetic anti-parallel (SyAP) pinned layer having an AP2/Ru/AP1configuration wherein the AP2 layer is made of CoFe and the AP1 layer iscomprised of Co_(1-X—Y)Fe_(X)B_(Y) having a thickness from about 10 to80 Angstroms wherein x is from about 5 to 95 atomic % and y is fromabout 5 to 30 atomic %.
 9. The method of claim 7 wherein said free layeris Co_(1-W)Fe_(W)/Ni_(1-Z)Fe_(Z) wherein w is from about 10 to 90 atomic% and z is from about 5 to 70 atomic % or is a multilayered structure ofalloys comprised of at least two of the elements Co, Fe, Ni, and B. 10.The method of claim 7 wherein the seed layer is Ta/Ru, Ta, Ta/NiCr,Ta/Cu, or Ta/Cr and the AFM layer is one of IrMn, MnPt, NiMn, OsMn,RuMn, RhMn, PdMn, RuRhMn, or MnPtPd.
 11. A method of forming a TMRsensor in a magnetic device, comprising: (a) depositing a first Mg layeron a ferromagnetic layer; (b) forming a MgO layer on said first Mglayer; and (c) depositing a second Mg layer on said MgO layer to form aMg/MgO/Mg tunnel barrier layer.
 12. The method of claim 11 wherein theMgO layer is formed by a rf-sputtering or reactive sputtering process.13. The method of claim 11 wherein the MgO layer is formed by a NOXprocess comprising an oxygen pressure between about 0.1 mTorr and 1 Torrfor a period of about 15 to 300 seconds, said NOX process partiallyoxidizes the first Mg layer.
 14. The method of claim 11 wherein theferromagnetic layer, first Mg layer, and second Mg layer are depositedby a DC sputtering process.
 15. A TMR sensor in a magnetic device,comprising: (a) a ferromagnetic layer; and (b) a tunnel barrier layerformed on said ferromagnetic layer, said tunnel barrier layer iscomprised of a lower Mg layer, a middle MgO layer, and an upper Mglayer.
 16. The TMR sensor of claim 15 wherein said ferromagnetic layeris a SyAP pinned layer having an AP2/Ru/AP1 configuration wherein theAP2 layer is made of CoFe, and the AP1 layer is about 10 to 80 Angstromsthick and is comprised of Co_(1-X—Y)Fe_(X)B_(Y) wherein x is from about5 to 95 atomic % and y is from about 5 to 30 atomic %, said TMR sensoris further comprised of a free layer formed on said upper Mg layer. 17.The TMR sensor of claim 15 wherein said ferromagnetic layer is a SyAPpinned layer having an AP2/Ru/AP1 configuration wherein the AP2 layer ismade of CoFe, and the AP1 layer has a multilayer structure comprised ofCoFeB/Co_(1-V)Fe_(V) wherein v is from about 10 to 90 atomic % and thethickness of the Co_(1-V)Fe_(V) layer is from about 5 to 20 Angstroms.18. The TMR sensor of claim 16 wherein the free layer is comprised ofCo_(1-W)Fe_(W)/Ni_(1-Z)Fe_(Z) wherein w is from about 10 to 90 atomic %and z is from about 5 to 70 atomic % or is a multilayered structure ofalloys comprised of at least two of the elements Co, Fe, Ni, and B. 19.The TMR sensor of claim 15 wherein the first Mg barrier has a thicknessof about 4 to 14 Angstroms and the second Mg layer has a thickness fromabout 2 to 8 Angstroms.
 20. The TMR sensor of claim 15 wherein the MgOlayer has a thickness that is adjusted to provide a MR ratio of at least34% and a RA value of about 2 ohm-um².
 21. The TMR sensor of claim 15wherein said magnetic device is a MRAM or read head and the TMR sensorhas a bottom spin valve configuration in which a Ta/Ru seed layer, IrMnAFM layer, CoFe/Ru/CoFeB pinned (ferromagnetic) layer, said barrierlayer, a CoFe/NiFe free layer, and a cap layer are sequentially formedon a substrate.